Fermi-LAT detection of the first bright gamma-ray flare from the Gigahertz-peaked Spectrum radio source OS 300

ATel #13931; C. C. Cheung (NRL), on behalf of the Fermi Large Area Telescope Collaboration
on 10 Aug 2020; 16:57 UT
Credential Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed gamma-ray flaring activity from a source positionally consistent with the radio source OS 300 (B1600+335, 4C +33.38), also known as 4FGL J1602.1+3324 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), with coordinates R.A. = 240.5302642 deg, Decl. = +33.4480756 deg (J2000; Johnston et al. 1995, AJ, 110, 880). OS 300 is best known as a gigahertz-peaked spectrum (GPS) radio source (Snellen et al. 2000 MNRAS 319, 445) with complex radio morphology (Tremblay et al. 2010 ApJ 712, 159). Its redshift was estimated photometrically as z = 1.1 (Snellen et al. 2000 MNRAS 319, 445) and is thus tentative (see Dallacasa et al. 2013 MNRAS 433, 147).

Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on 2020 August 9, with a daily averaged gamma-ray flux (E>100MeV) of (0.6+/-0.1) X 10^-6 photons cm^-2 s^-1, and a peak 6-hr flux of (1.0+/-0.3) X 10^-6 photons cm^-2 s^-1 from 12:00-18:00 UTC (statistical uncertainties only). The former corresponds to a flux increase of a factor of 130 relative to the average flux reported in the fourth Fermi-LAT catalog (4FGL). This is the highest LAT daily flux ever observed for this source. The corresponding daily photon index is 1.9+/-0.2, and is significantly smaller than the 4FGL value of 2.3+/-0.1.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source is being added to the "LAT Monitored Sources" and consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT will be publicly available ( http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/ ). We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is C.C. Cheung (Teddy.Cheung at nrl.navy.mil).

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.